This invention relates to a piezoelectric knock sensor for an internal combustion engine and specifically to such a sensor including an element adapted to resonate at a frequency characteristic of knock induced vibrations in the engine with which it is used. Such a sensor may be used in a system to measure knock in an internal combustion engine for the purpose of engine testing or fuel knock rating; or it may be used in an active closed loop knock control in which a knock affecting engine parameter such as engine spark timing is varied in response to the signal from said sensor to maintain knock at or below trace level.
There are many resonant piezoelectric knock sensors commercially available at the present time, almost all of them a self resonant design in which a circular piezoelectric element vibrates with substantially all its energy in a single resonant vibration mode at a predetermined frequency. The term "self resonant" is used herein to denote a resonant sensor in which the resonant frequency is a characteristic of some portion of the sensor alone and is not affected by the mass or vibration characteristics of the element on which the sensor is mounted. A sensor for which the resonant frequency depends in part on an interaction between the sensor and the object on which it is mounted is called an interactive sensor; and one example of such a sensor is shown in the U.S. Pat. No. 4,254,354 to John E. Keem, issued Mar. 3, 1981 and assigned to the assignee of this invention.
Knock occurs in an internal combustion engine when the spark ignited flame front of normal combustion within the combustion chamber compresses the unburned fuel mixture to the extent that this unburned mixture ignites spontaneously and generates acoustic cavity vibrations in the combustion chamber. These vibrations, which are dominated by certain audio resonant frequencies determined by the acoustic cavity resonance modes of the combustion chamber at the time of the knock event, cause the engine structure to vibrate in a similar manner and thereby cause the audible knocking or pinging sound for which the phenomenon is named. It has long been known that knock in a particular engine is generally characterized by one or more characteristic frequencies; and many proposed and actual knock measuring and control systems in the prior art make use of this fact in differentiating knock from other audible noises produced in engine operation. However, what has been very little discussed in the patent and other literature of the prior art and dealt with in a practical way by very few actual knock measuring or control systems is the fact that these characteristic knock frequencies are not firmly fixed for different specimens of the same basic engine, or for different cylinders of a single engine, or even for the same cylinder under different engine operating conditions. Since these frequencies are determined by the acoustic cavity resonances of the combustion chamber at the time of the knock event, they will thus vary somewhat with the volume of the combustion chamber at that time; and this volume will vary with the slightly varying dimensions of the engine parts within production tolerances as well as such combustion chamber volume and combustion characteristic parameters as spark timing, air-fuel mixture, engine inlet air temperature and atmospheric pressure, to name a few. Thus, in designing a practical knock detection and control system, one must be careful not to make any component of the system too narrowly resonant at a particular frequency.
The typical commercially available self resonant piezoelectric knock sensor generally comprises a mounting stud which may be attached to the engine or some component thereof, a case, a circular plate with piezoelectric voltage generating means and means for centrally or peripherally supporting the plate within but isolated from the case for self resonant vibration. The plate is supported in a symmetrical manner so that practically all of its vibrational energy appears in its first resonance mode, in which the periphery or center vibrates back and forth axially. This gives the sensor a sharply tuned, high Q resonance at a designated frequency, which is matched to the supposed knock frequency of the engine. Unfortunately, the sensor characteristic is so narrowly tuned that it does not sense all knock induced vibrations in all cylinders of each engine on which it is mounted at all times and therefore imparts a certain inconsistency of operation to the system in which it is used. Simple methods of broadening the response of the sensor such as the use of mechanical or electrical damping have the further undesirable effect of reducing the resonance peak so far that very little of the frequency selective advantages of resonance are attained. It would be desirable to produce a piezoelectric knock sensor which is mechanically resonant across a broader range of frequencies than the typical high Q resonant sensor while maintaining the high selectivity between those signals within the passband and those without. This kind of resonant characteristic is shown by the interactive knock sensor described in the aforementioned Keem patent; however, many designers would prefer to work, if possible, with a self resonant sensor, since the resonant frequency is determined solely by the resonating element and there is thus more freedom to design the case and electrical connectors of the sensor from the standpoint of minimum material and cost without concern for the effects of the design on the interactive resonant behavior of the sensor.